Electron transfer in Redox equation

  1. 1. The problem statement, all variables and given/known data
    How many electrons are transferred in the following reaction?
    6 Br^- (aq) + Cr2O7^2- (aq) + 14 H^+ --> 2Cr^3+ (aq) + 7H2O + 3Br2 (l)

    a) 1
    b) 2
    c) 3
    d) 6
    e) 14


    2. Relevant equations



    3. The attempt at a solution
    Redox reactions really confuse me, but I gave it a shot. I took a look at the number of electrons on each side of the equation and found the following:
    Left side:
    6Br^- = -6
    Cr2O7^2- = -2
    14 H^+ = +14
    Total: +6

    Right side:
    2Cr^3+ = +6
    7H2O = 0
    3 Br2 = 0
    Total: 6

    But this is as far as I can get. Somehow I don't think the answer is D) 6, but I really don't know. Someone please help me understand this problem?
     
  2. jcsd
  3. symbolipoint

    symbolipoint 3,043
    Homework Helper
    Gold Member

    Easily concentrate your attention to just one half-reaction, assuming the reaction as written is balanced; that being for the bromide. How many electron difference is it from 2 bromides to one bromine(compound, not the separate atoms)? Now, how many electrons change is this for the same half-reaction as represented in the fully written reaction (in which you start with 6 bromides instead of just 2 bromides)?
     
  4. symbolipoint

    symbolipoint 3,043
    Homework Helper
    Gold Member

    In other words, from post #2, your best choice would seem to be D. (6 electrons)
     
  5. Okay, so D it is. If I understand correctly then in the half-reaction for bromide... there is 1 electron difference between 2 bromides to 1 bromine. As a whole reaction, there are 6 bromides... so 6 electrons.

    What about the other half-reaction, though?
    Cr2O7^2- + 14H^+ --> 2Cr^3+ +7H2O

    I suppose it is the dichromate ion that confuses me.
     
  6. symbolipoint

    symbolipoint 3,043
    Homework Helper
    Gold Member

    If you know your reaction as written is balanced, then you know that the chromate part of the reaction is also for 6 electrons.

    Look at dicrhromate anion. Account for all the charges which give the -2 charge for this anion. Two chromiums, seven oxygens, the sum of the charges must be -2. You want to find the charge on the chromium. You know: 1 oxygen is -2, dichromate anion charge is -2. You do not yet know the charge on 1 chromium in the dichromate anion. This is what you want to find (using simple algebra).
     
  7. Ahhh... I see. Thank you for your help. I understand now. :) Redox reactions make my head spin.
     
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